High voltage electric power cables or power transmission cables are used to transmit electric power with medium or high voltage. The cables comprise a conductor and an insulation system that surrounds the conductor. Electric power cables that can be buried into the ground are called land cables. Electric power cables that can be buried into a sea bed which can freely extend between two fixing points in sea water are called submarine, sea water or underwater power cables. Underwater power cables are used today in an increasing amount due to the increased need of power transfer from for example offshore energy sources, including offshore renewable energy plants, such as wind power plants. Also the length for power transfer cables is increasing since there is a need to interconnect different regional electrical transmission networks to allow global trading of energy. Areas where energy is on the other hand needed and on the other hand produced may also be remote from each other which further increase a need for safe power transfer.
Aluminium conductors have been found to be suitable alternatives to traditional copper conductors in electric power cables, also called power transmission cables. Aluminium conductors are suitable for high voltage DC (HVDC)-cables, which are adapted to voltages of 50 kV or more. Long power transmission cables are produced by first joining short lengths or portions of conductors together to obtain a desired final length. The joints should not negatively affect the electrical and mechanical properties of the conductor. An insulation system and protective layers are subsequently applied onto the conductor to provide a power transmission cable.
There have been attempts to improve joints between the conductor portions. JP2012022820 describes a prior art method for providing a flexible joint for stranded conductors by welding a joint at each concentrically twisted layer and by welding the entire conductor element wires. However, materials for welding are not discussed in the document. Thus, despite known technologies, cost-effective and simple joining methods and materials are still needed.
Further in the prior art solutions the welding material, i.e. the welding metal rod or wire that melts during welding, has been similar material as in the conductor. This has been advantageous since the electrical properties of the cable have not been negatively affected by the joints. However, when using the same material as both welding material and the conductor material the welding process and the weld or joint obtained will become very sensitive. Sensitive welds or joints are a problem especially in underwater power cables, since the power cables are subjected to substantial external forces in the sea bed. Also, sensitive joints may brittle or even brake already before laying when the cable or conductor is coiled in storage coils. The coil diameter during coiling needs to be as small as possible and thus the cables or conductors are subjected to substantial bending forces which may break the joints. Therefore, there is a great desire to improve the mechanical strength of joints between conductor portions to provide stable and robust conductors.